The glass transition temperature of crystalline isotactic polypropylene (iPP) of around 0° C. limits the applicability of all iPP-based materials in the sub-zero temperature range. Combining iPP as a matrix phase with an elastomeric component having a sufficiently low glass transition temperature (Tg) is a standard approach for overcoming this problem. Even then, however, the performance at temperatures below around −10° C. is often limited.
Conventional heterophasic polypropylene polymers are based on a matrix phase and a partly amorphous C3/C2 phase. Heterophasic polymers can be formed using heterogeneous Ziegler Natta catalysis and such polymers typically comprise a crystalline PE phase as well as the matrix phase and partially amorphous phase. In contrast, single site catalyst produced heterophasic copolymers have a matrix phase and largely amorphous EPR phase. These polymers however, suffer from a number of design problems.
One problem is the dispersion of the elastomeric component in the matrix, due in part to the particle size of the elastomeric phase. Problems can also arise due to the viscosity ratio between elastomeric component and matrix (PP) phase, and the compatibility between these two phases. Incompatibility is a result of the compositional differences between the materials. Good compatibility is often achieved at high propylene (C3) content (and hence low ethylene (C2) content) in the rubber phase which, however, leads to a higher Tg, again limiting the performance at very low temperatures, such as below −10° C.
Attempts to increase the elastomer content (i.e. the EPR phase) to thereby improve impact strength will necessarily reduce the stiffness or tensile modulus of the polymer. Also, increasing the ethylene content inevitably reduces the heat resistance of the polymer.
The C3/C2-ratio in the disperse elastomer phase therefore defines both the glass transition point Tg of the ethylene propylene rubber (EPR) phase and the compatibility with the matrix component, the latter co-defining the particle size.
The inventors have also found that a certain molecular weight limit (frequently expressed as intrinsic viscosity (IV(EPR)) has to be overcome for the elastomer phase to effectively increase the impact strength, whilst too high molecular weight will both reduce the overall flowability of the composition and again increase the particle size.
The present inventors sought the production of relatively high flow heterophasic copolymers having MFR2 values of at least 0.5 g/10 min. At these high flow values, there are issues with property balance, e.g. in terms of impact strength and toughness. This invention offers heterophasic copolymers with an excellent balance of properties in terms of toughness and impact strength at high flow. These properties are achieved at commercially relevant glass transition temperatures Tg.
In particular, the present inventors have found that certain heterophasic propylene polymers having a rubber phase which is both at least partially crystalline whilst also possessing a very high ethylene content can have high weight average molecular weight (Mw) and hence offer attractive mechanical properties.
Similar polymers to those of claim 1 are known in the art. In EP-A-1,511,803, heterophasic copolymers are disclosed with high flow but with low ethylene content in both the polymer and the EPR phase thereof.
EP-A-2,053,086 generally describes Ziegler Natta based heterophasic copolymers with a 60-90 wt % of the matrix component and 10-40 wt % EPR component. C2 contents within the EPR phase are generally low.
WO2013/007650 and WO2013/007664 also describe heterophasic polypropylene resins comprising a propylene homopolymer matrix and an ethylene-propylene copolymer phase dispersed within the matrix with excellent low temperature impact properties. The polymers disclosed are however of low flow and the viscosity of the EPR phase is always lower than the matrix.
WO2009/077032 describes heterophasic copolymers with a relatively low viscosity xylene insoluble fraction containing high amounts of propylene monomer units within the rubber phase.
WO2012/028252 describes heterophasic polypropylene polymers with an amorphous xylene soluble matrix component having no more than 70 wt % ethylene in that matrix. These polymers have low viscosity.
WO2009/077034 describes heterophasic propylene polymers which can have high ethylene content within the rubber phase. However, this document generally describes polymers with a low xylene soluble content and low melting enthalpy of the polyethylene component (Hm(PE)). The catalyst used in this document does not produce crystalline fractions at the high ethylene content in the xylene soluble fraction in WO2009/077034. That is also reflected in its low Tg.
The present inventors sought polymers with high flow and good impact properties. In order to prepare the copolymers of the invention, the use of single site catalysis is required. The inventors have found that the process and catalysts described herein are ideal for the production of heterophasic propylene/ethylene copolymers as defined herein. This can be achieved with high productivity and high catalyst activity. Moreover, we have produced polymers that are stiffer at comparable impact strength.
As we note below, the catalysts used in the polymer manufacture are not themselves new and other similar catalysts are known. WO2009/054832 discloses conventionally supported metallocene catalysts which are branched at the 2-position of the cyclopentadienyl ring in at least one of the ligands making up the catalyst.
WO2007/116034 describes metallocene compounds substituted in the 2-position by a linear alkyl group. In particular the compound dimethylsilyl(2-methyl-4-phenyl-5-methoxy-6-tertbutylinden-1-yl dichlorozirconium is described which carries a methyl group at the 2-position.
WO2006/097497 describes certain symmetrical metallocenes based on tricyclic ring systems (tetrahydroindacenyl).
WO2011/135004 and WO2011/135005 describe rac-Me2Si(2-Me-4-Ph-5-OMe-6-tBuInd)2ZrCl2 but only in the context of propylene homopolymerization.
The complexes used in the process of the invention are however described in the WO2013/007650 and suggested for propylene ethylene copolymerisation. However, their use explicitly in the production of the propylene ethylene copolymers as herein described is not known.
It has now surprisingly been found that the particular complexes described below in solid form but free from external carrier can be used in propylene ethylene polymerisation with excellent results. They enable the formation of the heterophasic propylene ethylene copolymers described herein.
Moreover, the present inventors sought a polymer made in a three step process based on a first slurry step followed by two gas phase steps. The problem with such a set-up is catalyst activity in the third reactor in the sequence as catalyst must have a long enough lifetime to have acceptable activity in the third reactor (GPR2), in which the rubber phase is produced. Additionally, the inventors sought high molecular weight capability in GPR2 to give better mechanical properties at both room temperature and low temperature.
We have surprisingly found that by increasing the ethylene content of the rubber phase to between 70 to 90 wt %, both higher molecular weight and moderately crystalline rubber is achievable using the catalysts described herein. This component of the heterophasic copolymer can be prepared in the GPR2 reactor. The resulting polymers have a good mechanical profile, in particular a higher stiffness.